Introduction

The bulk of the earth's atmosphere (99% by mass) consists of molecular nitrogen and oxygen, which are homonuclear, diatomic molecules that absorb little radiation at wavelengths in the UV and visible spectral ranges. Trace amounts of polyatomic molecules including ozone are responsible for atmospheric absorption. In recent years, ozone loss has been tied to the release of man-made trace gases, mainly chlorofluorocarbons used in the refrigeration industry and as propellants in spray cans. Because ozone provides an effective shield against harmful UV solar radiation, a thinning of the ozone layer due to release of man-made trace gases (Solomon, 1990; Anderson et al., 1991) could have serious biological ramifications (Slaper et al., 1995; Herman et al., 1996; Zerefos and Bais, 1997; Herman et al., 1999).

UV radiation (280 nm - 400 nm) and visible solar radiation determine the concentration of photochemically active species through the process of photolysis, in which molecules are split up into atoms and smaller molecules. Here we assume for convenience that visible radiation is approximately equal to photosynthetically active radiation (PAR, 400 nm - 700 nm). Ozone is formed when an oxygen atom (O) and an oxygen molecule (O2) combine to yield O3. Chemical reactions and photolysis are responsible for the destruction of atmospheric ozone. The bulk content of the ozone gas residing in the stratosphere is determined by a balance between these production and loss processes.

UV radiation penetrating to the troposphere and surface is customarily divided into two spectral ranges: UV-B (280 nm - 320 nm) and UV-A (320 nm - 400 nm). Living organisms are much more susceptible to damage by UV-B radiation than by the more benign UV-A radiation. Fortunately, UV-B radiation is very effectively absorbed by ozone, and therefore very sensitive to the total column amount of O3.

Penetration of UV radiation and visible light in the ocean is strongly influenced by small plankton and thus, by biological productivity which provides a close link between biological and optical oceanography (Smith and Baker, 1989). Important aspects of the ozone depletion issue include the effects of increased UV radiation levels on algae, plankton, and fish larvae. Due to air-sea ventilation of dimethylsulfide (DMS), atmospheric sulfur compounds depend on the net production of DMS in the water column, and UV radiation photolysis of DMS represents an important removal process (Deal et al., 2005). Thus, since sources of atmospheric sulfur compounds are involved in cloud formation, the abundance of algae may indirectly affect atmospheric transmission, thereby linking atmospheric radiative transfer with ocean biology.

Major gaps in our present knowledge maybe illustrated by the following questions:

1. Which scattering and absorption agents of biogenic and non-biogenic origin are responsible for the scattering and absorption of UV radiation and visible light in the water column?

2. To what extent are current radiative transfer models able to predict the spectral UV radiation and visible light as a function of depth in the water column?

3. To what extent are current radiative transfer models able to predict the UV and visible spectral radiances emanating from the water column (the so-called water-leaving radiance), and transmitted to the top of the atmosphere?

4. What measurements are needed to test models of radiative transfer in an atmosphere-ocean system that includes an aerosol-loaded or cloudy atmosphere overlying open oceanic water or bodies of turbid water, such as lakes, rivers, and estuarine coastal waters?

5. What light measurements and modeling activities are required to validate remote sensing efforts aimed at characterizing biological productivity of water in the world's oceans as well as water quality of lakes, rivers, and estuarine coastal waters?

6. What is the minimum and what is the ideal number of spectral elements that need to be recorded by remote sensing instruments in order to accurately retrieve water properties in different water regimes?

Questions 1 and 2 are important because they are relevant to our basic understanding of the main drivers of biology in aquatic systems: the UV radiation and visible light fields. Lack of, or an incomplete understanding of, how UV radiation and visible light fields vary in an aquatic ecosystem, and how they relate to scattering and absorption properties of the water constituents, is a major obstacle to the prediction of how an aquatic ecosystem will function in a changing environment, e.g., how it will respond to expected changes, including global warming and ozone depletion. Attempts to assess the impact of UV radiation penetration on aquatic ecosystems have been made by several investigators as discussed by Hader et al. (1998), and efforts are currently underway to address the effect of stratospheric ozone variations on underwater UV irradiances on a global scale (Vasilkov et al., 2001) by making use of data from the Total Ozone Mapping Spectrometer and the Sea-viewing Wide Field-of-view Sensor (SeaWiFS).

Questions 3 - 6 must be addressed if we are to use the radiation reflected from the atmosphere-ocean system to retrieve information about biological productivity and water quality. The spectral dependence of the water-leaving radiance (i.e., the ocean color) carries information about the optical properties of the water column. If we know how these wavelength (color) dependent optical properties depend on the water constituents, we are in a position to retrieve important information about atmospheric parameters (e.g., aerosol type and optical depth) and marine parameters (e.g., concentrations of chlorophyll, dissolved and particulate organic matter, and suspended inorganic material) from measured water-leaving radiances (Stamnes et al., 2003; Li et al., 2008). Soil material, as well as man-made chemicals and agricultural fertilizing agents, are frequently transported into coastal and estuarine waters by rivers. Knowledge of how the optical properties of water are affected by such river discharges could be used to determine water quality if reliable models were available that related optical properties to water constituents of natural and anthropogenic origin. However, the retrieval of water-leaving radiances from measurements obtained with instruments deployed on earth-orbiting satellites requires accurate removal of the atmospheric component of measured radiances. This task is very difficult because the atmospheric aerosol optical properties vary rapidly, both spatially and temporally. The difficulty is compounded by the fact that 90% of the signal measured by downward-looking sensors in space comes from the atmosphere (Gordon, 1997), which means that accurate removal of the highly varying atmospheric contribution to a measured radiance becomes crucially important.

We start by providing a brief review of how UV radiation and visible light propagate throughout an atmosphere-ocean system. Then we give a few examples to illustrate possible applications of the theory: (1) comparing the results of two different radiative transfer models; (2) comparing measured and modeled radiation fields in sea ice; (3) discussing how multiple scattering in sea ice gives rise to radiation trapping; and (4) discussing how a polar ozone depletion might influence the primary production in icy polar waters.